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Taking Control of Screen Coating

(May 2009) posted on Wed May 20, 2009

The stencils you use for printing are only as good as the coating procedures you follow to produce them. Here you

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By KIWO Technical Support Department

Watching for the glass-like glossy finish to occur on the squeegee side, while coating from the substrate side, will indicate the first moment where the emulsion has filled the mesh completely. Each pass with the coater after this point causes the EOM to build. This change in thickness is fairly predictable.

To build up your EOM on the substrate side of the screen, simply turn the screen around and begin coating from the squeegee side. Each pass with the coater will continue to build wet emulsion thickness proud of (above) the mesh. How much the EOM builds with each pass depends in part on the coater design.

Drying After base coating, the screen is dried with the substrate side down and the squeegee side up. This allows gravity to pull emulsion to the substrate side of the screen where you want it. Not only will you build EOM, but you will also lower the Rz values.

Face coating The base-coating process allows you to lower Rz value at the same time that you’re building EOM, and the resulting screens are often perfectly useful for many applications. Unfortunately, printers who rely solely on wet-on-wet coating may end up with a stencil that is too thick before they reach an acceptable Rz value. The solution to this problem is face coating rather than trying to lower Rz through base coating.

Face coating is applied for the purpose of leveling the emulsion coating on the screen (Figure 4) (Figure Reference: Fine-detail printing often requires stencils that are both thin and have a low Rz to provide good gasketing during printing. To achieve such stencils, screens are base coated with only enough emulsion to completely fill the mesh openings and leave a slight EOM. After the screens are dried, a sharp-edge coating trough is used to coat the face of the screen, filling in any depressions between mesh knuckles so that the surface is uniform. The back (squeegee side) of the screen may also receive an additional thin coating of emulsion.) This reduces the Rz value and helps the stencil form a gasket during printing. Face coating is done after the base coating is dry. When face coating is performed with a sharp edged coater, it has the advantage of lowering the Rz with little change in stencil thickness (typically, less that a micron per coat).



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